Titration Strong Acid/Strong Base

Introduction

A titration is done with a strong acid and a strong base of equal concentrations using microscale techniques.

The chemicals used in this experiment were chosen because they react in a one-to-one stoichiometric mole ratio. Since you know precisely the concentration of base used, you can calculate the number of moles of base used. Because of the mole ratio, this is also the number of moles of acid used. Furthermore, from the volume of acid used, you can calculate the concentration or strength of the acid.

Since the same pipet was used for both acid and base, calibration of this pipet is unnecessary.

Macid = ( Mbase x # drops base)/# drops acid

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1. Select a Beral pipet to use for the reagents. Fill the pipet with distilled water and then squeeze dry. Fill the pipet with a small portion of acid. Expel in the sink. Fill again with acid and expel in the sink. Fill the pipet with acid.
2. Select a 24-well plate for the experiment. Carefully transfer 50 drops of acid to each of 3 wells. Add 1 or 2 drops of phenolphthalein indicator to each of the wells.
3. Squeeze the pipet dry. Rinse the pipet 3 times with small portions of distilled water. Discard the rinse at the sink. Fill the pipet with a small portion of base. Rinse. Expel. Repeat the rinsing procedure. Fill the pipet. (Instead, you may wish to use the two-piece device consisting of a plastic bulb and a pipet tip described above.)
4. Select one of the wells. Add base to this well, dropwise, while stirring, until a faint pink color is observed that remains for 30 seconds on standing. Record the number of drops required to reach this endpoint.
5. Repeat this procedure for the other 2 portions of acid.
6. Check your data. All three titrations should agree within 1 drop of one another. If they do not, repeat the titration.
7. Clean all your equipment and wash any left over acid or base down the sink.
   

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1. Suppose two different pipets had ben used for the acid and the base, what other experiments would need to be performed?
2. Must the wells be dry before you add the drops of acid? Why or why not?
3. Why is the phenolphthalein called an indicator?
4. Suppose the acid had reacted in a two to one ratio with the base. How would this change your calculations?
   

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Wear your safety goggles at all times during this experiment. Rinse spills immediately with large amounts of water. Have an eye wash available.

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1. The pipets would need to be calibrated to ensure that the drops were of equal size or to get data that permit adjusting for differences in drop size.
2. Since you are counting drops of acid and drops of base, water in the test tubes does not affect the lab.
3. It apparently indicates by changing color when enough base has been added to the acid.
4. When you calculated the moles of base you would need to multiply the result by two to find the moles of acid used.
   

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• If you want "true" results, standardize the base using a standard acid such as potassium hydrogen phthalate or oxalic acid. In any event, report the concentration of the base to your students using 3 significant figures.
• To check the validity of student results, titrate the HCl yourself using this technique.
• Most students will have little problem in getting reproducible trials. At first, the art of drop-by-drop addition is awkward. However, students quickly develop the skill and concentrate on forming and counting their drops.
• For uniform drops, try the device described in the special technique section in which the stem of a thin stem plastic pipet is cut to 1" and inserted into a plastic pipet tip.
  

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• micropipet (pulled Beral pipet).
• 24-well plate or 3 13 x 100-mm test tubes
• toothpicks
• 1% phenolphthalein indicator (dissolve 1 g phenolphthalein in 60 mL of 95% ethanol; add enough distilled water to bring the total volume to 100 mL.)
• 0.1 M NaOH (made by dissolving 4 g of NaOH in 1 L of distilled water).
• 0.1 M HCl (made by dissolving 9 mL of conc. HCl in 1 L of solution).

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The Woodrow Wilson microscale version of this lesson was developed by:

Kin Mack
Myers Park High School
2400 Colony Rd.
Charlotte, N.C.

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